The present invention relates to an aggregation reaction apparatus which is used for concentrating sludge or semi-liquid waste containing suspended solids, and separating liquid therefrom. Specifically, the present invention relates to a concentration type aggregation reaction apparatus which is capable of effecting an aggregation reaction simultaneously with concentrating sludge containing aggregates and separating liquid therefrom.
In a conventional method for removing suspended solids from sludge or semi-liquid waste, suspended solids in sludge are aggregated by various aggregation methods in a reaction tank, to thereby form pellets, and the sludge in the form of a mixture of pellets and a liquid is subjected to solid/liquid separation in a solid/liquid separation apparatus. By this method, when the concentration of suspended solids in the sludge is low, the volume ratio of the liquid supplied to the solid/liquid separation apparatus to the pellets becomes high and the throughput of the solid/liquid separation apparatus for separation of suspended solids becomes relatively low. As a method for increasing the throughput of the solid/liquid separation apparatus, there has been employed a method of providing a concentration apparatus between the aggregation reaction tank and the solid/liquid separation apparatus or a method of replacing the aggregation reaction tank with a concentration type aggregation reaction apparatus. As compared to the method of providing a concentration apparatus, the method of replacing the aggregation reaction tank with a concentration type aggregation reaction apparatus is inexpensive, and ensures efficient separation using a compact system.
In a concentration type aggregation reaction apparatus in which suspended solids in the sludge or semi-liquid waste are aggregated and the sludge containing pellets is concentrated and subjected to solid/liquid separation, the pellets grow so as to have a particle diameter of from about 2 to 10 mm. A liquid containing the pellets is partially passed through slits for separation provided in the reaction apparatus and discharged to the outside of the system, thus concentrating the liquid containing the pellets (suspension liquid).
FIG. 2 is a general vertical cross-sectional view of a typical example of a conventional concentration type aggregation reaction apparatus. Referring to FIG. 2, explanation is made with regard to how treatment of sludge is conducted by means of the concentration type aggregation reaction apparatus. In FIG. 2, a concentration type aggregation reaction apparatus 1 is substantially in the form of a vertical cylinder. A sludge feeding pipe 6 and a polymer feeding pipe 7 are connected to a lower portion of a side of the reaction apparatus 1. A concentrated sludge discharge pipe 9 and a separated liquid discharging pipe 19 are, respectively, connected to an upper portion and an intermediate portion of the side of the reaction apparatus. A driving device 12 is provided at an upper portion of the reaction apparatus 1. The driving device 12 revolves an agitating blade 15 and a scraper 21, which are connected to a shaft 14 so as to agitate the sludge in an aggregation reaction tank 2 of the reaction apparatus 1. A screen 3 having slits for separation of liquid is arranged in a form obtained by providing horizontally-positioned ring-shaped wedge wires vertically in a stacked configuration and fixing these wires by means of support bars. This structure of the screen is the same as that of FIG. 3 showing ring-shaped wedge wires 22 and support bars 23.
A draft tube 5 is provided in a central portion of the inside of the aggregation reaction tank 2, so as to increase an upward flow velocity on a surface of the screen 3. Sludge is agitated by the agitating blade 15 and flows upward along an inner wall surface of the aggregation reaction tank 2. While the sludge moves upward through a space between the inner wall surface and the draft tube 5 toward the screen 3, suspended solids in the sludge aggregate, to thereby form aggregate pellets. The pellets are subjected to solid/liquid separation at the slits of the screen 3. The liquid which has passed through the screen 3 is discharged from the separated liquid discharge pipe 19 to the outside of the system, thus increasing the concentration of the suspended solids in the sludge in the tank 2. The concentrated sludge is transferred from the concentrated sludge discharge pipe 9 to a solid/liquid separation apparatus. The separated liquid spontaneously flows, while a flow rate thereof is controlled by a telescope valve 18.
Thus, while the sludge is agitated by an agitator provided in the apparatus, aggregates such as aggregate pellets generally remain in the tank for several minutes and are transferred to a subsequent process in the form of concentrated sludge. However, the pellets remaining in the aggregation reaction tank are likely to adhere to and accumulate on the screen at the slits thereof, thus preventing discharge of the separated liquid. In this case, an effective area of the slits for separation decreases, thereby lowering efficiency of concentration. Therefore, adhesion and accumulation of the aggregate pellets is generally prevented by increasing a cross-flow velocity on a slit surface of the screen or providing a scraper 21 or a brush in the vicinity of the slit surface. From the viewpoint of simplification of a structure of the apparatus, it is desirable that mechanisms for preventing clogging of slits, such as those mentioned above, function without requiring a new driving apparatus. Therefore, in many cases, the mechanism for preventing clogging of the slits is imparted with a simple structure which utilizes a driving force of the agitator in the tank.
However, an effect of the clogging-preventing mechanism having a simple structure vary, depending on the properties of aggregates produced. Illustratively stated, the degree of adhesion of the aggregates to the slit surface of the screen varies and the mechanism cannot necessarily function as desired, depending on the viscosity of the aggregate pellets and liquid and the particle diameter and shear strength of the aggregate pellets. For example, the conventional clogging-preventing mechanism is satisfactorily effective when the pellets have a relatively large particle diameter and high shear strength, as in the case of treatment of sludge produced in a sewage disposal plant or an activated sludge treatment facility. However, when an aggregation reaction is conducted with respect to a special type of sludge containing particles having an extremely small diameter and having an extremely low content of inorganic matter, which inorganic matter affects the shear strength of the pellets, the pellets are likely to have extremely low shear strength, so that part of the pellets break and clog the slits. In this case, an effect of the clogging-preventing mechanism utilizing a cross flow or a scraper can be insufficient. That is, when the sludge particles or pellets have an extremely small particle diameter and low shear strength and are brittle, conventional concentration type aggregation reaction apparatuses cannot be suitably used.
Accordingly, it is an object of the present invention to obviate the above-mentioned disadvantages accompanying conventional techniques. It is an object of the present invention to provide a concentration type aggregation reaction apparatus which includes a mechanism for preventing clogging of slits of a screen by means of a simple structure, without using a driving device other than an agitator for an aggregation reaction, to thereby maintain stable efficiency of concentration.
In order to achieve the above-mentioned objects, the present invention provides a concentration type aggregation reaction apparatus having a concentration screen, which enables an aggregation reaction to be conducted in sludge and the sludge to be concentrated by separation of a liquid by means of a single apparatus. The concentration screen is in a cylindrical form and includes slits having horizontal openings. A decelerating device is connected to an upper portion of a shaft for an agitator for agitation in the tank and brushes are connected to the decelerating device. The brushes are adapted to revolve along opposite surfaces (on a primary side and a secondary side) of the screen. By use of these brushes, aggregates adhered to the screen are continuously removed. In the present invention, xe2x80x9cprimary sidexe2x80x9d means the side of the screen facing the sludge and xe2x80x9csecondary sidexe2x80x9d means the side of the screen facing a separated liquid.
The brushes may be arranged so that a clearance between tips of bristles of the primary-side brush and the primary side of the screen is 0 to 5 mm while a clearance between tips of bristles of the secondary-side brush and the secondary side of the screen is 0 to 10 mm, and the decelerating device may have a deceleration ratio of 1/5 to 1/15. By this arrangement, the sludge adhered to the screen can be efficiently removed.
It is most preferred that the clearance between the tips of the bristles of the primary-side brush and the primary side of the screen be 0 mm. However, the clearance of 0 mm is difficult to obtain in terms of manufacture. Therefore, the preferred range of the clearance between the tips of the bristles of the primary-side brush and the primary side of the screen is made 0 to 5 mm. With respect to the clearance between the tips of the bristles of the secondary-side brush and the outer side (secondary side) of the screen, due to the presence of support bars provided vertically on the secondary side of the screen, the amount of wear of the brush becomes large when the clearance is 0 mm. Therefore, it is preferred that the clearance between the tips of the bristles of the secondary-side brush and the secondary side of the screen be made about 0 to 10 mm.